runtime: wake select receivers for unbuffered sends

[cpunion]

Fixes #694.

Summary

• add a regression test for a goroutine blocked in select on an unbuffered receive while another goroutine starts a blocking send
• wake registered select receivers when an unbuffered sender transitions into the blocked-send state

Validation

• go test ./test -run TestSelectRecvWakesForBlockedUnbufferedSend -count=1
• /tmp/llgo-694 test ./test -run TestSelectRecvWakesForBlockedUnbufferedSend -count=1
• /tmp/llgo-694 run ./cl/_testgo/select
• /tmp/llgo-694 run ./cl/_testgo/selects
• repeated issue repro binary execution: 200/200 pass
• go test ./test -count=1
• /tmp/llgo-694 test ./test -count=1

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a concurrency issue where select-based receivers might not be woken up when an unbuffered channel sender becomes blocked. The fix involves explicitly notifying select operations (notifyOps) when an unbuffered send enters a blocked state, ensuring that waiting receivers can correctly process the pending send. A new regression test has been added to prevent future regressions of this specific scenario.

Highlights

• Unbuffered Send Wake-up: Implemented a mechanism to wake registered select receivers when an unbuffered sender transitions into a blocked state, ensuring proper channel communication.
• Regression Test: Introduced a new regression test (TestSelectRecvWakesForBlockedUnbufferedSend) to validate that goroutines blocked in select on an unbuffered receive are correctly woken by a blocking send.

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Changelog

• runtime/internal/runtime/z_chan.go
  • Added a call to notifyOps within the ChanSend function when an unbuffered channel's send operation blocks, ensuring select receivers are notified.
• test/select_test.go
  • Created TestSelectRecvWakesForBlockedUnbufferedSend to verify that a select receiver is correctly woken when an unbuffered send blocks.

Activity

• A regression test was added to cover the specific scenario of a goroutine blocked in select on an unbuffered receive while another goroutine starts a blocking send.
• The core fix involves waking registered select receivers when an unbuffered sender transitions into the blocked-send state.
• Validation steps were performed using go test and a custom llgo tool, including repeated execution of the issue reproduction binary, all passing successfully.

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[xgopilot]

Clean, well-targeted fix for a real race condition. The placement of notifyOps(p) after p.sends++ and before p.cond.Wait() correctly ensures select-based receivers are woken when a sender becomes available. The test is a good regression test. Left a couple of minor inline comments.

[gemini-code-assist]

Code Review

This pull request addresses an issue where select receivers were not being woken up for unbuffered sends. The fix in runtime/internal/runtime/z_chan.go correctly notifies select operations when an unbuffered sender is about to block, which seems appropriate. The new regression test in test/select_test.go effectively covers the scenario. I've identified a minor issue in the test where a goroutine could leak in case of a timeout and have provided a suggestion to make the test more robust.

_{Note: Security Review did not run due to the size of the PR.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.16%. Comparing base (eda01de) to head (194199a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1691   +/-   ##
=======================================
  Coverage   93.16%   93.16%           
=======================================
  Files          48       48           
  Lines       13314    13314           
=======================================
  Hits        12404    12404           
  Misses        724      724           
  Partials      186      186           

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